More specifically, the invention concerns an airship comprising a flexible envelope without a rigid internal structure imposing a shape on the envelope, said envelope delimiting an inside space intended to be at least partially filled with a lighter-than-air first gas, extending along a longitudinal axis between a nose located at the front of the airship and a tail located at the rear of the airship, and comprising an upper portion located at the top and a lower portion located at the bottom, said lower portion being opposite the upper portion.
The lighter-than-air gas contained in an airship expands when the airship gains altitude, and conversely it contracts when the airship approaches the ground.
Airships with a flexible envelope typically comprise one or more ballonets inflated with air, said ballonets being housed inside the envelope of the airship. These ballonets are gas-tight bags, usually pressurized by fans which draw outside air into these ballonets. The air pressure in the ballonets directly exerts pressure on the light gas filling the rest of the inside space defined by the envelope. The shape of the envelope is thus kept substantially constant regardless of the altitude of the airship. This shape of the envelope, in a plane perpendicular to the longitudinal axis, is usually circular.
The disadvantage of these airships comprising ballonets is that it is necessary to increase the volume of the envelope quite significantly, for example by about 30%, to accommodate the ballonet(s) within the inside space of the envelope. In addition, the pressure in the ballonets must be regulated in order to maintain the outer shape of the envelope.
Airships also exist that have a flexible envelope of varying external shape. Document FR-A-0 663 009 describes an example of such an airship. It comprises stretchable tethers inside the envelope which connect several points of the envelope. As the tethers connect three or four points distributed within a plane transverse to the longitudinal axis of the airship, the airship has a three-lobed or four-lobed shape when it is close to the ground, and a more or less cylindrical shape when higher in altitude where the lighter-than-air gas expands and stretches the internal elastic tethers. This airship with a flexible envelope does not comprise internal ballonets, but the shape of its envelope changes with altitude.
The disadvantage of these airships is that their shape, which depends solely on their altitude, is not controlled.